1. Field of the Invention
The present invention relates to wavelength monitors, which are typically used in the field of optical measurement technology to measure the wavelengths of light sources oscillating in a single mode.
2. Description of the Related Art
Light sources for single-mode oscillating DFB-LD (distributed feedback laser diode) and DBR-LD (distributed Bragg reflector laser diode) have a problem of experiencing drifts as they are used for a prolonged period of time, so in a DWDM (dense wavelength division multiplexing) system, the wavelengths of such light sources must be controlled by measuring them at appropriate times.
Wavelength tunable light sources of an external resonator type using a diffraction grating are extensively used to measure the wavelength characteristics of optical components. While they are capable of setting a desired value of wavelength over a broad range (≧100 nm), this type of light sources are sensitive to external effects and temperature changes in particular affect the wavelength stability. In addition, as the DWDM system becomes adaptive to higher degrees of multiplexing, it is required to increase the probability that the light source has stable wavelengths.
Among the conventional devices for measuring the wavelengths of light sources, the most commonly used are spectroscopes such as an optical spectrum analyzer that rotate a diffraction grating by a moving mechanism.
However, the moving mechanism in such spectroscopes makes them bulky and poses other problems such as limited long-term reliability. To deal with these difficulties, various types of wavelength monitor have been developed that measure the wavelengths of light sources at appropriate times with a compact design having no moving mechanism.
Among these wavelength monitors, one called a wavelength locker is used to control the wavelength of a DFB-LD light source with a structure that uses optical components such as interference film based filters and diffraction gratings.
The wavelength monitor called a wavelength locker can only be operated over a narrow wavelength range but using no mechanical moving parts, it has high reliability, can be reduced in size, requires no large-scale processing with software and, hence, is suited to the purpose of controlling the wavelengths of light sources such as one for DBF-LD that seldom vary in wavelength. There is, however, a problem in that the operating wavelength is limited by the wavelength characteristics of the components used such as filters.
A conventional wavelength monitor that is free from this problem may be a “wavelength change measuring apparatus” disclosed in JP 11-034697 A. A block diagram for the configuration of this apparatus is shown in FIG. 6 as a reference for the following description of the apparatus.
As shown in FIG. 6, the “wavelength change measuring apparatus” under consideration is basically composed of an input fiber 201, a collimating lens 202, a beam splitter 203 as a first splitting means, a first reflector 204, a second reflector 205 with a step of d=λ0/8, a reflecting prism 206 as a second splitting means, a first photo-detector 207, a second photo-detector 208, and a signal processing circuit 209 for processing signals from the two photo-detectors.
Measuring light emerging from the input fiber 201 is collimated and launched into the beam splitter 203 as the first splitting means provided on the axis of the emerging light, whereby it is split into two beams, one directed toward the first reflector 204 and the other toward the second reflector 205. The first reflector 204 and the second reflector 205 are provided normal to the optical paths of the split beams of collimated light emerging from the beam splitter 203 and their optical axes are adjusted such that each of the split beams of collimated light will be reflected back to the beam splitter 203 by travelling through the same optical path.
The second reflector 205 is a plane reflector having a step of d=λ0/8, so when a light beam is reflected by the second reflector 205 and travels through the return path, one half of the beam plane generates a difference of λ0/4 in pathlength (λ0 is the center wavelength in the measurable wavelength range and may take the value of 1550 nm).
The two beams of collimated light that have been reflected by the first reflector 204 and the second reflector 205 make a second entry into the beam splitter 203, where they are recombined before entering the reflecting prism 206 as the second splitting means,
The reflecting prism 206 is provided such that the axial plane where the pathlength difference of λ0/4 has been generated coincides with the edge tip surface of the reflecting prism 206; the recombined parallel light incident on the reflecting prism 206 is split into two beams by its edge tip surface and the split beams are launched into the first photo-detector 207 and the second photo-detector 208 which are provided on their optical axes. The light beams entering the two photo-detectors are output to the signal processing circuit 209 as currents that depend on their optical intensity. The signal processing circuit 209 compares the light intensities from the two photo-detectors and perform necessary arithmetic operations to output wavelength data.
The changes in light intensity vs. wavelength that are obtained with the ordinary Michelson interferometer are expressed by the following eq. 1. A normalized Gaussian distribution in a light beam plane gives a uniform light intensity, which varies uniformly with changing wavelength.I=[1+cos[4π*ΔL/λ]]/2  (1)
In eq. 1, I signifies the normalized light intensity received by a photo-detector; λ is the wavelength of the light input from the light source; ΔL is the pathlength difference in the Michelson interferometer.
In the design described above, a plane reflector having a step of d=λ0/8 is used as the second reflector 205, so when a light beam is reflected by the second reflector 205 and travels through the return path, one half of the beam plane generates a pathlength difference of λ0/4, thereby producing periodic interference light intensity signals with a phase difference of π/2.
Using the interference light intensity signals with a phase difference of π/2, one can determine the amounts and directions of changes in the wavelength of a light source.
However, the above-described conventional wavelength monitor, or the “wavelength change measuring apparatus” disclosed in JP 11-034697 A, does not satisfy the low-cost requirement since a step mirror, or a mirror specially designed to have a step of d=λ0/8, must be used.
As a further problem, diffraction occurs at the boundary between the step and the non-step area of the step mirror, producing distorted interference light signals.